1. Field of the Invention
The present invention relates generally to the art of electric storage batteries, for example automotive and truck batteries. More particularly, the invention relates to manifold vent caps and covers for such batteries which provide a flow path for the escape of hydrogen and oxygen formed during the electrochemical reactions which take place in such batteries, as well as resistance to acid spewing. Still more specifically, the invention relates to a vent cap manifold which directs the flow of gas to an explosion attenuation device and a cover design which retains electrolyte and ensures its return to the battery cells so that it will not become entrained in the flow of gases passing through the manifold or flow through the manifold to the attenuation exhaust port.
2. Description of the Prior Art and Technical Problems
Conventional lead-acid batteries, such as those used for automobiles and trucks, generally include a number of cells disposed in a battery housing. Each cell typically includes a plurality of positive and negative battery plates or electrodes, and separators are sandwiched between the plates to prevent shorting and undesirable electron flow during the reactions which take place during manufacture and use of the batteries. The plates and separators are immersed in a liquid electrolyte in the cells, the most common being aqueous sulfuric acid. The positive plate generally is constructed of a lead-alloy grid covered with lead oxide, while the negative plate generally contains lead as the active material, again covering a lead alloy grid.
In most battery constructions the battery housing includes a box-like base to contain the cells and which is made from a moldable resin. The housing is generally rectangular in horizontal cross-section, the cells being provided by vertical partitions within the housing. A cover is provided for the casing, the cover including terminal bushings and a series of filler holes to allow electrolyte to be added to the cells and to permit whatever servicing is required. To prevent undesirable spillage of electrolyte from the fill holes, most prior batteries have included some sort of filler hole cap.
The electromotive potential of each battery cell is determined by the chemical composition of the electroactive substrates employed for the electrochemical reactions. For lead-acid batteries, such as those described above, the potential is usually about two volts per cell, regardless of cell volume. Vehicles manufactured by original equipment manufactures (OEM's) typically require twelve volt batteries, so most of today's batteries include six cells (6 cells.times.2 volts per cell=12 volts). The size of the housing for the battery is selected for the "envelope" for a particular vehicle, i.e. the physical dimensions defined by the vehicle manufacturer for containment of the battery in the engine compartment.
Battery electrolyte spillage or spewing can be caused by a number of factors, including vibration or tilting as a vehicle maneuvers during normal use. Electrolyte escape may also be caused by battery overheating, a problem especially pronounced in recent years with smaller engines, which tend to run hotter than prior engines.
In addition to preventing spillage or spewing of electrolyte from the cells, the battery cover design and the filler caps need to perform an important and different function. This is because gases are liberated from lead-acid batteries during the charge and discharge reactions. Such reactions start at the time the battery is originally charged (called "formation") by the manufacturer or by the retailer or vehicle manufacturer. They also occur during normal operation of the battery. Factors such as high current charge and discharge conditions, and changes in temperature, can affect the rate at which gas evolution occurs. Control of gas generation and evolution in lead-acid battery construction is particularly important, because the gases are hydrogen and oxygen, and it is important to vent such gases in a controlled way from the battery to prevent pressure buildups in the housing which could lead to electrolyte leaks, housing failures, or most significantly explosions within the housing. It is also desirable, and well known, to prevent an external flame from entering the battery through gas exhaust ports.
As will soon become apparent, many prior art devices are known for venting gases from battery cells in a manner which allows diffusion of the potentially explosive hydrogen gas. It will also become apparent that prior attempts provide vent caps or covers with a flame or spark blocking material, generally known as an explosion attenuation element. However, it will also be seen that the focus of such prior art caps is on gas venting and the exhaust thereof through an explosion attenuation media.
The two problems previously mentioned, i.e. electrolyte spewing and gas evolution, are really interrelated and important in the construction of an effective cover and vent system. For example, electrolyte may enter the vent cap through several mechanisms. One mechanism is through vibrational or tilting flow of electrolyte into the cap, and another is through a mechanism frequently referred to as pumping. The latter occurs when gas evolved in the battery bubbles from the cells and carries or forces electrolyte out the fill hole and into the cap. When electrolyte enters the caps of some prior designs it may be carried out the exhaust passageway and cause damage to external battery components, such as the battery terminals or adjacent engine components.
Original equipment manufacturers are beginning to recognize the importance of the dual function performed by vent caps and covers and have instituted a number of testing specifications designed to ensure electrolyte retention in the cells. One such test involves tilting a battery 35.degree. about the longitudinal center line of the battery under vibration load in both directions. This test is quite severe and could not be passed by a number of the prior art batteries using the vent constructions referred to below.
In Hennen, U.S. Pat. No. 3,597,280, issued Aug. 3, 1971, a "Multiple Vent Plug Assembly" is described which includes three vent barrels entering three separate compartments, each of which is vented to the atmosphere. Circular baffles and other internal design features obstruct electrolyte to keep it from flowing to the vents.
Another patent issued to the assignee of the present invention is Hennen's U.S. Pat. No. 3,879,227 entitled "Battery Vent Plug." This ganged plug (multiple fill holes capped by a single vent cap or manifold) features downwardly directed barrels for the fill holes and conical or sloping bottoms around drain opening which themselves include a slanted point to facilitate dripping of electrolyte into the cells. Gases follow a tortuous path through a porous diffuser adjacent the gas outlet. Semicircular baffles also surround each opening into the vent cap to facilitate directing electrolyte to the lowermost tip of the drain barrels. The gas pathway through the diffuser is upwardly. In one embodiment an open bottom tube is suspended from the top of the vent cap housing and depends downwardly over and is spaced above the cell vent opening.
Different explosion attenuation devices for single cells are disclosed in Melone, U.S. Pat. No. 3,915,753, issued Oct. 28, 1975 and entitled "Liquid Indicator for a Storage Battery with a Flame Barrier Vent Filter" and Auerbach, U.S. Pat. No. 3,944,437, issued Mar. 16, 1976 entitled "Explosion Proof Venting Device for Electrical Storage Batteries." Both provide tortuous flow paths for gases leaving the battery. The former additionally provides a liquid level indicator, while the latter provides a catalyst in the diffusion material to assist in the recombination of hydrogen and oxygen gases generated within the battery.
Oxenreider, et al., in U.S. Pat. No. 4,278,742, issued Jul. 14, 1981 and entitled "Manifold Vented Battery Cover," also illustrates a battery cover employing a labyrinth design formed between two cover components which together form individual chambers for each battery cell, the chambers being interconnected by ports.
Other explosion attenuation vent caps are described in commonly owned U.S. Pat. No. 4,916,034, issued Apr. 10, 1990 to Hulsebus, et al. and entitled "Battery Vent Strip." In this device, a vent cap includes a series of barrels with a strip extending transversely to the line of barrels, the strip including a porous explosion attenuation material. A plurality of channels couple the cells to the flame arrestor. Splash guards are provided to reduce electrolyte leakage into the exhaust flow path and the flame arresting material.
A different type of cap is shown in commonly assigned U.S. Pat. No. 5,284,720, issued Feb. 8, 1994 to Thuerk, et al. and entitled "Vent Cap With Electrolyte Drain And Explosion Attenuation Capabilities." This device includes a vent cap having a sloping floor drain, a baffle system and a gas entrance for the attenuation device located above the battery centerline.
One current design of battery cover used for truck batteries includes a molded cover having six holes, one for each battery cell. Horizontal holes within the cover interconnect the head space above each cell to allow gases to pass toward an explosion attenuation device. The holes are prepared during the molding process by passing a rod through the mold. Individual, screw-in type filler caps are employed for each of the openings. The cover further includes an internal passageway for coupling these holes, through a baffle and leading to an explosion attenuation device. The passageway includes a slanted floor and a single opening and a single exit. The device is better than many designs which are currently in use but suffers from manufacturing difficulties due to problems resulting from the rod/mold combination. Moreover, the screw-in caps do not provide the advantages of gang type systems, and the passageway design is not entirely efficient in returning acid to the battery in certain tilt orientations.
While a number of different solutions have been proposed in the aforementioned patents to the technical problems discussed earlier in this section of the specification, optimization has still not been achieved, especially in batteries which employ centerline terminal posts.
An improved vent cap and cover construction for minimizing the possibility of electrolyte leakage from the battery and for inhibiting the introduction of sparks or flame into the battery and efficiently directing gases from the battery would represent a substantial advance in this art.